Wind power is growing at a fast pace worldwide. The Doubly-Fed Induction Generator (DFIG) is widely adopted in wind turbines because of its variable speed operational capability, low operating noise, mechanical stress mitigation, and control flexibility for active and reactive power. Fault ride-through (FRT) capability is essential for power grid-connected DFIG-based wind generation to permit continued operation through severe grid voltage disturbances. Even if faults are far away from the turbine, serious voltage dips could be induced at the connection point. According to recent grid codes, large wind farms should stay connected to High Voltage (HV) grids when faults occur, since disconnection may further degrade voltage restoration during and after fault conditions.
In DFIGs, the rotor windings are connected to the grid via back-to-back converters that control the rotor current and inject current into the grid. The system is sensitive to grid disturbances since the stator winding connects to the grid and rapid change of stator voltage “traps” flux, leading to large induced voltage in the rotor windings at rotational frequency. Therefore, proper protection schemes and FRT techniques are necessary in order to withstand grid faults. Active “crowbars” are commonly utilized to protect the rotor-side converters (RSC) against voltage and current transients caused by voltage dips in the stator side by shorting the rotor winding. When the crowbar is engaged during a fault, the DFIG behaves like an induction machine since the rotor winding is short-circuited by shunt resistors and the RSC is disabled. An obvious drawback of employing the crowbar that the DFIG consumes reactive power and might worsen the grid voltage during the faults. Series dynamic resistors (SDR) may also be utilized to restrain significant rotor currents so that the RSC and rotor circuits can be effectively protected by way of coordination control of the chopper resistor and the crowbar. However, if the wind farm is connected to a weak power network, employing such techniques makes the DFIG unable to supply substantial amounts of reactive power and increases the chance of system instability. Static synchronous compensators (STATCOMs) have also been proposed to supply additional reactive power and compensate for DFIG consumption, but STATCOMs may be inadequate to prevent the system from either rotor overcurrent or dc-link overvoltage. To achieve full system protection, STATCOMS are usually combined with other protective elements such as, for example, stator braking resistors and crowbars, which increase system complexity, cost, etc.
Another possible solution to the FRT problem uses a dynamic voltage restorer (DVR) to isolate the DFIG from the connection point in case of voltage dips. The DVR is a voltage source converter (VSC) that is connected to the grid via a series transformer. During grid faults, the DVR compensates for voltage variations, allowing the DFIG to not deviate from normal operation. However, the DVR solution is very costly for full voltage compensation because it requires the VSC to have the same capacity as the DFIG. A series grid-side converter (SGSC) topology has also been proposed. Studies of the SGSC solution show good FRT performance and emphasize the potential of using series compensation topologies for DFIG applications, but dedicated SGSC converters also increase system complexity and result in higher system cost.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.